U.S. Pat. No. 4,629,676 (Hayakawa et al) and European Patent Publication No. 0174634B1 disclose an image forming method comprising the steps of imagewise exposing to light a light-sensitive material that comprises a support and a light-sensitive layer containing silver halide, a reducing agent and a polymerizable compound, and simultaneously or thereafter heating the light-sensitive material to develop the silver halide and harden the polymerizable compound. The silver halide, the reducing agent and the polymerizable compound are preferably contained in microcapsules that are dispersed in the light-sensitive polymerizable layer. According to the method, a hardened image is easily formed on the light-sensitive material by the heat development. Further, the sensitivity of the light-sensitive material is very high because silver halide is used as a photosensor.
The heat development is convenient and advantageous compared with a wet development using a developing solution. However it is rather difficult to control the conditions of the heat development (e.g., temperature, time and heating means). For example, the hardness of the obtained image is sometimes insufficient when the light-sensitive material is heated while the surface of the light-sensitive layer is open to the air. Oxygen in the air has a function of inhibiting a polymerization reaction. The most convenient way of preventing the influence of oxygen is to heat the light-sensitive material while pressing the surface of the light-sensitive layer on the surface of a heating means.
U.S. Pat. No. 4,945,025 (Nakamura) discloses a light-sensitive material having a cover sheet. The sheet covers the surface of the light-sensitive layer to prevent the influence of oxygen.
By the way, the above-mentioned image forming method is available for forming a lithographic plate or a color proof.
U.S. Pat. No. 5,122,443 (Takeda) and European Patent Publication No. 0426192B1 disclose a method of forming a lithographic plate. Further, U.S. patent application Ser. No. 07/878,996, which is now U.S. Pat. No. 5,304,454, and German Patent Publication No. 4215035A1 disclose a method of forming a color proof. In the method of forming a lithographic plate or a color proof, a cross-linkable polymer may be used in place of or in combination with the polymerizable compound. Further, the polymerizable compound or the cross-linkable polymer is uniformly contained in the layer without use of microcapsules.